PURPOSE: To prevent continuous operation of a machine while connecting a part of batteries reversely to other batteries by comparing the initial voltage of power supply, immediately after conduction thereof, with a set level and controlling an electronic switch ON/OFF.

CONSTITUTION: A voltage comparator 4 compares the initial voltage of a power supply 1 comprising more than two batteries connected in series, immediately after conduction thereof, with a set level and delivers an ON signal if the set level is exceeded otherwise delivers an OFF signal to an electronic switch 5. The set level is equal to the voltage level when one of three batteries connected in series is connected reversely. This constitution prevents continuous operation of a machine while connecting a part of batteries reversely by detecting the inadvertent reverse connection of the part of batteries at the initial stage of conduction between the power supply 1 and a load 2.